Now that President Clinton has decided to take on the issue of teen pregnancy, he has a tricky problem: How to avoid sounding like a Republican.

The president and his staff are putting together a plan for a major initiative aimed at reducing the number of illegitimate births, especially to young women who haven't finished high school.

Even if it does give Republicans an opening to say told-you-so, Clinton has good reasons to take on illegitimacy from his bully presidential pulpit. The growing rate of out-of-wedlock births to teen-age mothers causes or exacerbates several major social and economic problems he has promised to remedy.

Single mothers, especially those who start having children before they get a high school education, are a major cause of poverty in this country and a major reason why it is so difficult to reduce the numbers of the poor.

It will be hard, if not impossible, for Clinton to fulfill his job training plans and other initiatives to "end welfare as we know it," if the number of poor, single-parent families keeps growing.

Crime, to which Clinton is forced to pay attention by increasing public concerns, often has roots in dysfunctional families in poor urban areas, where so many children are growing up without fathers or good male role models and often see no realistic, more attractive alternatives. Drug use and drug pushing flourish in this social environment. So do gangs, with their high potential for violence.

How well schools succeed generally reflects the educational and economic level of the parents of students and what they learn at home. Schools burdened with large numbers of children whose only parent is a high school dropout tend to be among the lowest achieving in the nation. Government can not be a missing parent. Unmarried teen-agers are more likely than other mothers to have high-risk pregnancies and premature babies who need costly neonatal care.

If more girls postponed pregnancy until they are married, self-supporting and ready for parenting, it would mean significant savings in health care - another Clinton initiative.

The facts are hard to ignore. In 1990, 28 percent of American babies were born to unmarried women, compared to 5 percent in 1960. That's a total of 1,165,384 children born without a legal father, a 75 percent increase since 1980. Sixty-six percent of all black infants are born to unmarried mothers. Of the more than 1 million teen-agers who become pregnant this year, more than half of the pregnancies will be ended by abortion or miscarriage. Two-thirds of the 490,000 babies who are born will have no legal father.

The consequences of unmarried motherhood are most severe for teen-agers who have not graduated from high school before they have a baby. Almost 80 percent of them are poor and many will be stuck in poverty for years, if not for generations.

But the Democrats derided the Republicans when they tried to sell the nation on "family values" and the benefits of marriage as part of their 1992 political strategy. Republicans were tarred as stuffy, moralizing, sexist, punitive and out of touch with reality. Democrats poked fun for months at Dan Quayle's remark about Murphy Brown being a poor role model.

What can President Clinton do differently?

Whatever he tries, Clinton will probably get some of the same flak that hit the Republicans: Times have changed; we can't go back to '50s morality. Inner city culture is different; why try to impose middle-class values? Don't pick on single mothers; they are doing the best they can and deserve respect and support. Children shouldn't be stigmatized or denied help because of their parents' marital status.

Clinton supporters are more likely than Republicans to be pushing school health clinics that distribute condoms rather than lectures about just-say-no abstinence.

They are talking about limiting welfare benefits to two years and debating what strings might be placed on the money to change behavior.

Like everyone else, they will stress job training, although prototype programs are expensive and show only limited and often temporary success. Changing social attitudes will complicate the effort. Unmarried parenthood is now supposed to be considered just another lifestyle, an alternative choice - not a problem. Popular entertainment celebrates unrestrained sex, without concern for consequences.

Religious sanctions have lost much of their power. Social strictures have faded away, pushed aside by the sex revolution and by our well-meant reluctance to stigmatize children as bastards and parents as sinners.

But economic incentives, at least in the immediate short-term, encourage teen pregnancy.

Welfare checks make it possible for a young, unmarried mother to have her own household and support her offspring without working and the money, in reality, relieves teen-age fathers of financial responsibility, whatever the child support laws say.

The president's bully pulpit, however skillfully Clinton uses it, isn't likely to have much effect on the rate of unmarried teen pregnancy, without significant changes in social and economic incentives. But both Democrats and Republicans should wish him success. So should everyone who cares at all about children.